Interconnection structure made of redirected carbon nanotubes

ABSTRACT

The invention relates to an electronic device including electric connections extending along at least two different directions, said connections being essentially formed by means of bundles of carbon nanotubes (CNT) ( 8 ), where at least two CNT bundles comprise a portion ( 8   a ) having its axis directed along a first direction and a portion ( 8   b ) having its axis redirected along a second direction, the connections between CNT bundles being achieved by overlapping of the portions ( 8   b ) of said at least two bundles to form a connection line ( 4 ).

TECHNICAL FIELD

The present disclosure relates to an electronic device comprisingelectric connections formed by means of carbon nanotubes (CNT), it alsorelates to methods for forming such connections.

This invention especially has applications for thermal, electric, andmechanical connectors.

BACKGROUND OF THE INVENTION

The use of carbon nanotubes (CNT) or of CNT bundles to manufacturethrough vias or chip interconnects has already been provided, especiallyto provide a complement or even an alternative to the use of copper,indeed, the latter is not adapted when minute sizes are involved, CNTsfurther have the necessary properties, such as a low electricresistance, enabling to provide the best electric conductivity betweenthe different chip levels.

A via is a cavity enabling to create a connection between conductiveplates. The electric lines formed on the plates create the connectionbetween vias. Conductive plates are made of a metal, such as aluminum,and are separated by an insulating layer having the cavity forming thevia dug therein.

The miniaturization of electronic devices makes the use of copper quiteproblematic since copper causes difficulties due to electromigrationwhen current densities become too high. Architectures formed of copperlines and vias thus show their limits in integrated circuits having aresolution close to 22 nanometers.

As illustrated in FIGS. 1 and 2, the use of CNTs for ensuring theconnection between conductive tracks has been provided, by replacingcopper or tungsten vias with CNTs (Katagiri et al., interconnectTechnology Conference, 2009, IEEE international 1-3 Jun. 2009, pp.44-46; Yokoyama et al. Japanese Journal of Applied Physics, Vol. 47, No4, 2008, pp. 1985-1990). However, this method does not enable to totallydo away with copper, the connection between vias being always ensured bya copper line. Electromigration-related issues are thus not totallysuppressed.

Document US 200810042287 describes an electronic device in which theconnections are at least partially ensured by CNT bundles. The via iscovered with a layer of conductive material on which another CNT bundlecan be deposited and directed along the line direction. However, thevias and the lines are not formed from the same CNT bundle.

Document US 2006/0212974 discloses an electronic device comprising CNTbundles prepared inside of the via, and then redirected along anotherdirection to connect two conductive layers of two different levels.There are no connections between vias.

Document CN 101562148 relates to a method for creating vertical. CNTconnections by deposition of a CNT solution on a conductive layer. Inthis device, two conductive layers of different levels are connected bymeans of CNT bundles.

Another technology is based on the same concept of CNT vias, butcomprises using metal blocks to change the orientation of the CNTs andthus form the horizontal lines (FIG. 3). It is however difficult tocontrol the deposition of a catalyst and the CNT growth along twoperpendicular directions on two surfaces of a metal block (FIG. 4).Another disadvantage of this technique is the repeated crossing of manyCNT—metal interfaces to ensure the conduction.

Document US 2009/0294966 describes vertical CNT vias ensuring theelectric connection between two conductive layers, but also horizontalCNT lines providing an electric connection between vias. This involvestwo separate bundles directed along two different directions. The CNTbundles coming from the via do not enable to form the line.

These different approaches imply controlling the CNT growth withinsmaller and smaller cavities, thus raising the issue of the CNT bundledensity. Indeed, just like the homogeneity of their properties and oftheir orientation, the control of the CNT density is of major importanceto provide good electric connections in nanoelectronics. High CNTdensities are thus indispensable.

Hata and co-workers (Hayamizu et al., Nature nanotechnology, Vol. 3,2008, 289-294) have recently revealed an organizing and densifyingeffect capable of being obtained by dipping of a film of scattered. CNTsin an alcohol solution. Indeed, when the CNT film is dipped into thealcohol bath, perpendicularly to the surface thereof, and then dried,the CNTs gather and align. Due to the surface tension of the liquid andto the strong Van der Waals interactions, the CNTs achieve a structureclose to that of graphite. No disassembly of the CNTs has been observedafter densification. However, Hata only obtains structures formed ofCNTs directed along the same direction, which thus limits possibleapplications.

The present invention comes from the search for technical solutionsespecially enabling to do away with the use of metals and implementingsimple manufacturing processes.

SUMMARY OF THE INVENTION

Thus, the present invention provides a new architecture, enabling to doaway with the use of metals to ensure the connection between plates orto ensure the change of direction of a CNT bundle, which is based on thegrowth and the redirection of carbon nanotubes (CNTs).

Generally, the present invention comprises forming the electricconnections, in an electronic device, with bundles of carbon nanotubes(CNT) directed along a first direction and contained in cavities calledvias. Such CNT bundles are laterally interconnected by lines, alsoformed of CNT bundles along a second direction.

In the rest of the discussion, terms “via” and “line” are used todesignate the CNT bundles contained in the cavities or vias andrepresenting the connection lines, respectively.

Typically, an electronic device according to the present inventioncomprises a sequence of structures especially made of a conductive plate(for example, made of aluminum) covered with an insulator layer (silicaor low-K microelectronics material). Cavities interconnected by linesare dug into the insulating block to create interconnects between plates(vias) or between vias (lines).

More specifically, the present invention relates to an electronic devicecomprising electric connections extending along at least two differentdirections. Typically, said connections are essentially formed by meansof bundles of carbon nanotubes (CNT), where at least two CNT bundlescomprise a portion having its axis directed along a first direction anda portion having its axis redirected along a second direction. Further,the connection between CNT bundles is achieved by overlapping of theportions of said at least two CNT bundles to form a connection line.

At least two bundles forming the electric connections are bent, an areaof the CNT bundles being directed along the first direction and anotherarea being directed along a second different direction.

it should be noted that the electric connection system of the presentinvention enables to form connections in at least two directions,advantageously vertical and horizontal, but may also be used to createconnections in more than two directions, especially three, in particularin the case of two different connections in the horizontal plane.

According to the present invention, the electric connections are mainlyformed by means of CNT bundles or bunches, that is, a multitude ofaggregated carbon nanotubes having a substantially parallel growth axis.Term “essentially” indicates that the electric connections may be onlyensured by the CNTs, and thus in the absence of metal lines or blocks,as used to be the case in prior art. However, and as will describedhereafter, the electric contact between bundles may be improved byfurther performing a metal deposition.

In practice, such electric connections are created by implementing thefollowing method:

-   -   growth of at least one CNT bundle along the first direction;    -   redirection of a portion of the CNT bundle along the second        direction, advantageously by flowing of a liquid.

Typically, the method for creating electric connections in at least twodirections within an electronic device according to the presentinvention comprises the steps of:

-   -   growing at least two CNT bundles along a first direction in a        cavity of said device;    -   redirecting a portion of said two CNT bundles along a second        direction, advantageously by flowing of a liquid, to form the        connection line.

The forming of this device thus comprises the controlled growth of CNTbundles along a first direction within cavities, according to techniquestried and tested in prior art, especially by means of catalysts such asiron.

Typically, the growth of the CNT bundles is stopped when said bundleshave a height at least greater than that of the via. A portion of theseCNT bundles is then redirected along a second direction, advantageouslyby flowing of a liquid.

The redirection of the CNT bundle may be performed by the techniquedescribed in the Hayamizu et al. document. (Nature nanotechnology, Vol.3, 2008, 289-294). In practice, it comprises immersing the CNTs in anisopropyl alcohol solution and pulling them parallel to the direction ofa groove, the groove being perpendicular to the meniscus of the liquid.This operation also enables to densify the CNT bundles.

According to a privileged embodiment, the portion of the CNT bundlewhich has been redirected along the second direction is substantiallyperpendicular to the portion of the CNT bundle along the firstdirection.

Typically, the first direction is substantially vertical.Advantageously, the portion of the CNT bundle along the first directionforms the via of the electronic device.

Preferentially, the second direction is substantially horizontal.Advantageously, the portion of the CNT bundle along the second directionforms the connection line of the electronic device.

In a privileged embodiment, vias and lines are thus substantiallyperpendicular.

To form connections between vias, in particular, the electronic deviceaccording to the present invention preferentially comprises at least twoCNT bundles having their portions along the second direction, which maybe substantially horizontal, forming the connection line. The line isadvantageously formed by overlapping of the portions of the CNT bundlesalong the second direction, that is, by successive superposition of thebundle ends.

This embodiment is implemented by the following method:

-   -   growth of a plurality of parallel CNT bundles along the first        direction, advantageously within vias;    -   redirection of a portion of the CNT bundles along the second        direction;    -   connection, advantageously by overlapping, of the portions of        the CNT bundles along the second direction, to form the        connection line.

The overlapping is actually obtained by flattening of the upper portionof the CNT bundles, thus only leaving the portion of the CNT bundlecontained in the cavity in the first direction, preferably substantiallyvertically. Advantageously, the second direction, that is, theoverlapping direction, is that of a groove previously dug into theinsulating block to contain the connection line.

According to a specific embodiment, the device of the present inventionfurther comprises a metal layer. Advantageously, it covers at least theportion of the CNT bundles along the second direction, and moreadvantageously still the connection line.

This metal layer may enable:

-   -   to uniformize the surface of the device for an encapsulation or        to be used as a support to form an upper connection level. In        this last case, it is advantageous to deposit a metallic        material, for example, aluminum, capable of promoting the growth        of a second CNT level; and/or    -   promote the connection between vias.

This metal layer is deposited by means of any adapted technique known bythose skilled in the art.

According to another embodiment, the electronic device may furthercomprise another CNT bundle laterally connected to at least one CNTbundle of the device, at the level of its portion in the seconddirection, to form the line. This other CNT bundle ensures theconnection with a bundle coming from a via and possibly between severalvias. The connection is achieved after the growth of this other CNTbundle, either directly along the second direction, or after itsredirection along the second direction, as described previously.

Preferentially, the other CNT bundle is laterally connected to at leasttwo CNT bundles to ensure the connection between vias.

According to the selected operating mode, the growths of CNT bundlesforming either the vias, or the connection lines, may be simultaneous orseparate.

Advantageously, the growth of the other CNT bundle is performed within agroove and not within a cavity. The other CNT bundle thus cannotdirectly come into contact with the conductive plate, since it isphysically separated therefrom by the insulating block. The redirectionof this other CNT bundle along the second direction may be performed inthe groove.

In this specific embodiment, the lateral connection between CNT bundlesis advantageously achieved by the deposition of a metal layer. Inpractice, it is located at the interface of the two types of CNTbundles: at least one CNT bundle, at the level of its portion along thesecond direction, coming from the via(s) and the other bundle formingthe line. This layer is advantageously formed by means of a so-calledcontact metal selected from the following group:

palladium, copper, gold, or titanium. Thus, the contact metal ensuresthe connection between the CNT bundles coming from the vias and the CNTbundle forming the connection line.

In the context of the present invention, to locate CNTs in predeterminedgrowth areas, one may:

-   -   either deposit the growth catalyst over the entire plate, and        then remove it (by etching, polishing . . . ) from the        non-predetermined areas;    -   or deposit TiN on the non-predetermined areas, then deposit the        catalyst fall wafer, as described in Dijon et al. (Diam. Relat.        Mater., 2009, doi:10.1016/j.diamond.2009.11.017).

According to a preferred embodiment, the method for creating electricconnections in at least two directions within an electronic deviceaccording to the present invention further comprises a step ofdeposition of at least one metal layer on the portion of the CNT bundlesalong the second direction.

It may also comprise a step of growth, and possibly of redirection alongthe second direction, of another CNT bundle intended to laterallyconnect at least one CNT bundle, at the level of its portion along thesecond direction, to form the connection line. In this specific case,the method may possibly comprise a step of deposition of a metal layeron the other CNT bundle.

Further, the method for forming the device according to the presentinvention may comprise at least one step of deposition of TiN on theareas where CNTs should not grow.

In electronic devices according to the present invention, the electriccurrent appears to essentially flow through the carbon nanotubes, thusconsiderably decreasing electromigration problems. Further, methods forforming such devices implement relatively tried and tested techniques.

EMBODIMENTS OF THE INVENTION

The foregoing and other features and advantages of the present inventionwill be discussed in the following non-limiting description of thefollowing embodiments in connection with the accompanying drawings.

FIG. 1 shows a diagram illustrating the integration of carbon nanotubes(CNT) in prior art ULSI (“Ultra Large Scale integration”) interconnects.

FIG. 2 is a diagram illustrating prior art, that is, the growth ofcarbon nanotubes (CNT) in vias, creating a contact with the coppercables.

FIG. 3 is a three-dimensional microscope view of a prior art line/viainterconnection system, implying a metal contact block.

FIG. 4 is a diagram of a prior art line/via interconnection system,implying a metal contact block.

FIG. 5 is a diagram of the interconnection device according to thepresent invention, by overlapping of redirected CNTs coming from vias(A) or by connection between separate CNTs (B), respectively. The arrowindicates the current flow and the circles indicate the interfaces to becrossed.

FIG. 6 is a diagram of two interconnection devices according to theinvention (A, B), where the contact between CNT bundles is improved bymeans of a metal.

FIG. 7 is a schematic diagram of an embodiment of the inventionaccording to which the vias and lines are formed simultaneously.Left-hand views: cross-section views; right-hand views: top views.

FIG. 8 is a diagram illustrating a method for forming an interconnectiondevice according to the present invention by CNT overlapping; A/ to F/:end views, perpendicular to the groove; G/ and H/: views parallel to thegroove direction.

FIG. 9 is a diagram illustrating a method for forming an alternativeinterconnection device according to the present invention bysimultaneous growth of the vias and of the line.

FIG. 10 is a diagram illustrating a method for forming an alternativeinterconnection device according to the present invention by independentgrowth of the vias and of the line, and insertion of a contact metal.

DETAILED DESCRIPTION OF THE INVENTION

The different embodiments described hereafter relate to an electronicdevice requiring both vertical and horizontal interconnects, thus in twodifferent perpendicular directions. All these connections are formed bymeans of carbon nanotuhes (CNT):

-   -   the vertical connections are formed by vertical CNT bundles        which grow within vias, formed in a layer of insulating        material;    -   the horizontal connections are also ensured by CNTs and form the        connection lines. They may result from the overlapping of the        bundles coning from the vias, said bundles having been        redirected (first embodiment; FIG. 5A) or result from the growth        of an independent bundle in lateral contact with the bundles        coming from the vias, said bundles having been redirected        (second embodiment; FIG. 5B).

According to a specific embodiment, a metal layer 2, 10 is also incontact with the CNT bundles, especially on portions 8 b of the CNTbundles (FIG. 6A and FIG. 6B).

Such a layer may advantageously be formed of

-   -   a layer 10, for example, made of Pd or Ti, thus improving the        contact of CNTs; and/or    -   a layer 2, for example, made of Al, allowing the growth of a new        CNT level, and thus of a new interconnection level.

As a variation, it may be a bilayer associating the two types of layers10 and 2.

I/ First Embodiment of the Invention

As already mentioned, this first embodiment by overlapping of verticalCNT bundles is illustrated in FIGS. 5A and 6A, respectively.

More specifically, its forming method is illustrated in FIG. 8. Itshould be noted that steps A to F are schematically shown in end views,that is, perpendicular to groove 3, FIGS. 8G and 8H corresponding toviews parallel to the direction of groove 3.

A/ Forming of the Base Structure:

An insulating layer 1, made of silica or of a low-K material ofmicroelectronics is deposited on conductive layer 2. Conductor 2typically is aluminum.

B/ Etching of the Groove:

Within insulator 1, groove 3, which will become future line 4, is formedby conventional lithography methods.

C/ TiN Deposition:

A TiN layer 5, having a thickness of approximately 50 nanometers, isdeposited in groove 3, if possible by means of a conformal depositionmethod. TiN has the function of inhibiting the growth of carbonnanotubes when catalyst 6, here iron, is deposited on the TiN.

D/ Via Opening:

Vias 7 are opened in groove 3, the etching through insulator 1 stoppingon conductor 2.

E/ Catalyst Deposition:

The deposition of catalyst 6 is performed at ambient temperature. Ittypically is a layer of 1 nanometer of iron, deposited by evaporation orby ion beam sputtering. The deposition is performed with a normalincidence, to minimize the side coverage.

F/ Selective Nanotube Growth:

Conventionally, carbon nanotubes (CNT) 8 are grown at 600″C, by means ofa C₂H₂+H₂+He mixture (10 sccm, 50 sccm, 50 sccm), after havingpreviously oxidized the iron by means of an RF air plasma formed atambient temperature. The plasma conditions are the following:

-   -   P=0.3 Torr;    -   70-W power for 30 minutes.

This method allows the growth of nanotubes 8 on iron 6, except if thelatter is deposited on TiN 5. In this case, there is no growth.

The pressure during the 600′C growth is 1 Torr. The reactive gases areintroduced after the cold plasma and the temperature rise is performedwithin 15 minutes with a 0.3-Torr pressure.

The height of nanotubes 8 is set by the growth time.

G/ Nanotube Redirection:

After the growth, the device is immersed in isopropylic alcohol andpulled perpendicularly to the direction of groove 3: the meniscus of theliquid is perpendicular to groove 3. The liquid flowing through thegroove tubes flattens tubes 8 coming from vias 7. The tubes are highlydensified after this operation. Further, tubes 8 have two differentportions:

-   -   a portion 8 a, having an axis parallel to that of vias 7, and    -   a portion 8 b, having an axis parallel to that of groove 3.

Nanotube bundles 8, coming from the different vias 7, are thus flattenedin groove 3 and come into contact. It is thus possible to form a lineconnection 4 by overlapping of at least two nanotube bundles coming fromdifferent vias 7.

At the end of this step, an interconnection system such as illustratedin FIG. 5A is obtained. Line 4 is formed by overlapping of nanotubebundles 8 b coming from vias 7. In this embodiment, the line resistanceR is formed of a series of interface resistances added to the nanotuberesistance.

However, in a subsequent step (FIG. 8H), it may be recommended toperform a metal deposition 2, again with aluminum, at the surface ofline 4 to planarize the structure and to be able to repeat the operation(creation of interconnects) at the next level (encapsulation). At theend of this step, an interconnection system such as illustrated in FIG.6A is obtained.

II/ Second Embodiment of the Invention

This second embodiment is illustrated in FIGS. 5B, 6B, 7, 9, and 10,

FIG. 5B illustrates the fact that the line resistance is lower than inthe first embodiment since there is only one interface resistance left.

FIG. 7 is a drawing illustrating the principle of this second embodimentaccording to which the substantially perpendicular interconnects,respectively vias 7 and lines 4, come from at least two differentbundles of nanotubes, advantageously formed simultaneously. It should benoted that the diagrams of the left-hand portion show cross-sectionviews while the right-hand diagrams are top views.

Further, the embodiment implying the simultaneous growth of nanotubes 8and 8′ from line 4 and vias 7 is illustrated in FIG. 9, which is derivedfrom FIG. 8.

Steps A to C are similar to those of FIG. 8.

However, before the opening of vias 7, an additional step is carried out(FIG. 9D′): an opening 9 of TiN 5 is formed in groove 3, to obtain thegrowth of carbon nanotubes 8′ in this area after deposition of catalyst6. This corresponds to the arranging of growth area 9 of the line.

The next step comprises openings vias 7 after having protected openings9 with a resin layer (FIG. 9E′).

After deposition of catalyst 6 (FIG. 9F′) and during the growth step(FIG. 9G′), tubes 8 and 8′ grow in vias 7 and in growth area 9 formed atthe line level, respectively. The bundles coming from CNTs 8 and 8′ maybe given a different length, by varying the catalyst thickness and thedifferent plasma conditions in the vias and on the line.

After redirection of the two nanotube bundles (8, 8′) coming from vias 7and from growth area 9, respectively, the obtained interconnectionsystem corresponds to that illustrated in FIG. 5B. It can be observedthat connection line 4 is formed by means of a nanotube bundle 8′ whichdoes not come from vias 7.

FIG. 10 derives from FIG. 8, but in an embodiment where the growth ofnanotubes 8 and 8′ is performed separately from line 4 and vias 7, andwhere an interface metal 10 is inserted between the two nanotube bundles(between the CNTs of vias 8 b and the CNTs of lines 8′, respectively).

The method starts as in steps A to G of FIG. 8.

However, instead of step H, steps H′ to M′ of FIG. 10 are implemented:

H′/Deposition of a Contact Metal:

After redirection of nanotubes 8 coming from vias 7 (8 a, 8 b), contactmetal 10, such as palladium, copper, gold, or titanium is deposited.

I′/Contact Metal Opening;

Contact metal 10 is then opened by etching at the end of groove 3 andall the way to insulator 1. Growth area 9 of the future nanotubes 8′coming from line 4 is thus formed.

J′/ Catalyst Deposition:

The catalyst is deposited by evaporation or sputtering.

K′/ Contact Metal Planarization:

The device is planarized by CMP (“Chemical and Mechanical Polishing”) tosuppress catalyst deposit 6 on contact metal 10, while keeping catalyst6 in growth area 9.

L′/ Second Nanotube Growth:

A second growth in the same conditions as at step F of FIG. 8 isperformed. The bundle of nanotubes 8′ coming from growth area 9 andintended to form line 4 is thus obtained.

M′/ Line Redirection:

After the growth, the nanotube bundle is redirected in the same way asat step G of FIG. 8. A connection line 4 in contact via metal 10 withnanotubes 8 coming from via 7 is thus obtained.

At the end of such a process, an interconnection system such asillustrated in FIG. 6B is obtained.

1-13. (canceled)
 14. An electronic device comprising electricconnections extending along at least two different directions, saidconnections being formed by means of carbon nanotube (CNT) bundles,wherein at least two CNT bundles each comprise a portion having an axisdirected along a first direction and a portion having an axis redirectedalong a second direction, the connections being achieved by connectingportions of said at least two CNT bundles to form a connection line. 15.The electronic device of claim 14, wherein the at least two differentdirections are substantially perpendicular.
 16. The electronic device ofclaim 14, wherein the first direction is substantially vertical and theCNT bundle portion having an axis directed along the first directionforms a via.
 17. The electronic device of claim 14, wherein the seconddirection is substantially horizontal and the CNT bundle portion havingan axis redirected along the second direction forms a connection line.18. The electronic device of claim 17, wherein the connection linecomprises a metal layer.
 19. The electronic device of claim 14, whereinthe electronic device further comprises another CNT bundle laterallyconnected to at least one of the at least two CNT bundles, at theportion of the CNT bundle having an axis redirected along the seconddirection, to form the connection line.
 20. The electronic device ofclaim 19, wherein the other CNT bundle is laterally connected to atleast two CNT bundles to ensure the connection between vias.
 21. Amethod of creating electric connections extending along at least twodirections of the electronic device of claim 1, the method comprising:growing at least two CNT bundles along a first direction in a cavity ofsaid device; and redirecting a portion of said at least two CNT bundlesalong a second direction by flowing a liquid to form a connection line.22. The method of claim 21, wherein: growing at least two CNT bundlesalong the first direction is carried out within vias; a portion of theat least two CNT bundles is redirected along the second direction; andthe portions of the at least two CNT bundles redirected along the seconddirection are connected by overlapping to form the connection line. 23.The method of claim 21, comprising depositing at least one metal layeron the portion of the at least two CNT bundles redirected along thesecond direction.
 24. The method of claim 21, comprising growing, andoptionally redirecting along the second direction, another CNT bundle toconnect laterally at least one CNT bundle to the portion of the CNTbundle redirected along the second direction, to form the connectionline.
 25. The method of claim 24, comprising depositing a metal layer onthe other CNT bundle.
 26. The method of claim 21, comprising depositingTiN on areas where CNT bundles should not grow.